Neurostimulation has been increasingly used as a therapy in areas where conventional pharmacological approaches become ineffective, such as in treating refractory pain, Parkinson disease, dystonia, and obsessive compulsive disorder. The stimulated structures may vary greatly. For example, in treating neuropathic pain, the neurostimulation techniques include transcutaneous electrical nerve stimulation (TENS), peripheral nerve stimulation (PNS), nerve root stimulation (NRS), spinal cord stimulation (SCS), deep brain stimulation (DBS), epidural motor cortex stimulation (MCS), and repetitive transcranial magnetic stimulation (rTMS) 1. A specific example is using MCS to treat trigeminal neuropathic facial pain, a syndrome of severe, constant pain due to pathological change or injury to the trigeminal system 2. The target is located anterior to the central sulcus and posterior to the splitting of the inferior frontal sulcus.
Most neurostimulation methods are invasive and rely on electrodes that are implanted into the stimulated structure through intraoperative surgery. A pulse generator and a battery must be implanted in the body, often causing complications such as infection, lead migration, hardware malfunction, battery failure or unwanted stimulations. It was reported that, overall, 43% of patients experience one or more complications3. Significant research has been conducted on the use of rTMS, a non-invasive approach for brain stimulation. In rTMS, electric fields are induced by a fast changing, magnetic flux to cause the stimulation. The electric field penetration depth is limited to a 2 cm4, and the excitable volume is not less than ten cubic centimeters 5. Besides electrical stimulation, ultrasound energy was used for neurostimulation studies 6, aiming to improve the focus of the stimulation and to access deeper brain zones, but the significant difference in the sonic properties between the brain tissue and the bone, as well as the irregular skull shape prevent the focusing of the power7. Recently, optogenetic approaches revolutionized the neurostimulation using light, but the expression of photon-sensitive ion channels is required and the implantation of a light source is needed8.